1. Field of the Invention
The present invention relates to a method for purification of exhaust gas from an internal-combustion engine. In more detail, the present invention relates to a method for purification excellent in removing particulates and the like from exhaust gas of an internal-combustion engine, in particular, a diesel engine or the like, and also in removing sulfur compounds and the like in a NOx storage catalyst.
2. Description of the Related Art
A diesel particulate filter is used to collect PM (particulate matter) such as black smoke or SOF or the like, exhausted from an internal-combustion engine like a diesel engine or the like; which poses a problem of deposition of PM in the filter and thus increases pressure loss, accompanying with use thereof.
To overcome this problem, regeneration of the filter has conventionally been executed, by arrangement of a heating apparatus such as an electric heater or the like at the filter, so as to combust and remove PM by heating (Japanese Patent No. 2953409).
However, such a regeneration method had a problem of increased power consumption and high running cost. In addition, space occupied by a heating apparatus as well as a filter is required, which posed a problem of limitation in mounting place for mounting the apparatus in a vehicle. To solve these problems, an purification apparatus for diesel exhaust gas, provide wit ha filter for collecting diesel engine particulates, a catalyst converter with good activity for hydrocarbon (HC), mounted at the upstream side of an exhaust gas pipe of the filter, and a HC control unit which is capable of supplying a large quantity of HC to the catalyst converter has been proposed, and as the catalyst, platinum, palladium, rhodium or the like is disclosed (JP-A-60-043113).
In addition, a filter for diesel exhaust gas particulate coated with catalyst components thereon, is also disclosed (U.S. Pat. No. 5,100,632).
Furthermore, such a purification system of exhaust gas from an internal-combustion engine has also been proposed that is provided with a continuous regeneration type diesel particulate filter at an exhaust gas passage of an internal-combustion engine, for removing particulate substances collected, by executing a regeneration mode operation, when collected amount of the particulate substances in the filter for collecting the particulate substances is equal to or higher than a predetermined determination value, wherein the system being provided with a collecting amount prediction unit for predicting collected amount of particulate substances collected in the filter, and a maximal injection amount control unit for controlling the maximal injection amount of an internal-combustion engine, when the particulate substances predicted by the collecting amount prediction unit is equal to or higher than the predetermined determination value (JP-A-2004-108207).
In addition, it is known that, in a method for removing nitrogen oxide, catalytic performance is reduced by accumulating sulfur oxides like SOx to a NOx storage catalyst used in removal of NOx or the like, and a regeneration method, also in recovering performance thereof, by introducing a large quantity of hydrocarbon to raise temperature has been proposed, however, this method limits a temperature range suitable to regeneration (Japanese Patent No. 3747639).
In addition, it is disclosed that exhausted hydrocarbon can be efficiently purified at a low temperature region at engine start, by using a catalyst supporting different amount of catalyst and catalyst components at the inflow side and the outflow side on the same catalyst. This catalyst has features in that HC is temporarily adsorbed by a HC adsorbing material at low temperature where catalytic activity is not exerted, like in an engine start region, and HC is purified when catalytic activity is exerted by temperature increase, and in that oxygen release amount is set higher in the upstream side than in the downstream side. And, it is desirable that content of a noble metal is higher in the outflow side than in the inflow side of exhaust gas. However, in the case where content of a noble metal is higher at the outflow side than at the inflow side of exhaust gas as described above, as shown in FIG. 6, effect of enhancing combustion performance of high concentration hydrocarbon cannot be exerted. In addition, in the present invention, as shown in FIG. 4 and 5, a catalyst providing higher decrease amount of HC concentration increases CO2 concentration more. These results indicate that a combustion reaction as shown in chemical equation (A) occurs, different from a temporal adsorption of HC (JP-A-2003-200049).
                              CxHy          +                                    (                              x                +                                  y                  4                                            )                        ⁢                          O              2                                      →                              x            ⁢                                                  ⁢                          CO              2                                +                                    y              2                        ⁢                                                  ⁢                          H              2                        ⁢            O                                              (        A        )            